The present invention relates to methods and systems for inconspicuously embedding binary data in line art images (such as are used in currency and the like), and associated methods/systems for decoding such data from such images. One application of such technology is discouraging counterfeiting of banknotes.
Watermarking is a quickly growing field of endeavor, with several different approaches. The present assignee""s work is reflected in U.S. Pat. Nos. 5,710,834, 5,636,292, 5,721,788, 5,748,763, 5,748,783, and 5,745,604, in allowed U.S. applications Ser. No. 08/327,426 (now U.S. Pat. No. 5,768,426), Ser. No. 08/508,083 (now U.S. Pat. No. 5,841,978), Ser. No. 08/438,159 (now U.S. Pat. No.5,850,481) and in laid-open PCT application WO97/43736. (The laid-open PCT application is substantially identical to the disclosure of U.S. Pat. No. 6,122,403.) Other work is illustrated by U.S. Pat. Nos. 5,734,752, 5,646,997, 5,659,726, 5,664,018, 5,671,277, 5,687,191, 5,687,236, 5,689,587, 5,568,570, 5,572,247, 5,574,962, 5,579,124, 5,581,500, 5,613,004, 5,629,770, 5,461,426, 5,743,631, 5,488,664, 5,530,759,5,539,735, 4,943,973, 5,337,361, 5,404,160, 5,404,377, 5,315,098, 5,319,735, 5,337,362, 4,972,471, 5,161,210, 5,243,423, 5,091,966, 5,113,437, 4,939,515, 5,374,976, 4,855,827, 4,876,617, 4,939,515, 4,963,998, 4,969,041, and published foreign applications WO 98/02864, EP 822,550, WO 97/39410, WO 96/36163, GB 2,196,167, EP 777,197, EP 736,860, EP 705,025, EP 766,468, EP 782,322, WO 95/20291, WO 96/26494, WO 96/36935, WO 96/42151, WO 97/22206, WO 97/26733. Some of the foregoing patents relate to visible watermarking techniques. Other visible watermarking techniques (e.g. data glyphs) are described in U.S. Pat. Nos. 5,706,364, 5,689,620, 5,684,885, 5,680,223, 5,668,636, 5,640,647, 5,594,809.
Most of the work in watermarking, however, is not in the patent literature but rather in published research. In addition to the patentees of the foregoing patents, some of the other workers in this field (whose watermark-related writings can by found by an author search in the INSPEC database) include I. Pitas, Eckhard Koch, Jian Zhao, Norishige Morimoto, Laurence Boney, Kinco Matsui, A. Z. Tirkel, Fred Mintzer, B. Macq, Ahmed H. Tewfik, Frederic Jordan, Naohisa Komatsu, and Lawrence O""Gorman.
The artisan is assumed to be familiar with the foregoing prior art.
In the following disclosure it should be understood that references to watermarking encompass not only the assignee""s watermarking technology, but can likewise be practiced with any other watermarking technology, such as those indicated above.
Watermarking can be applied to myriad forms of information. The present disclosure focuses on its applications to line art imagery, of the sort typically employed in banknotes, travelers checks, passports, stock certificates, and the like (hereafter collectively referred to as xe2x80x9cbanknotesxe2x80x9d). However, it should be recognized that the principles discussed below can also be applied outside this particular field.
Most of the prior art in image watermarking has focused on pixelated imagery (e.g. bit-mapped images, JPEG/IMPEG imagery, VGA/SVGA display devices, etc.). In most watermarking techniques, the luminance or color values of component pixels are slightly changed to effect subliminal encoding of binary data through the image. (This encoding can be done directly in the pixel domain, or in another domain, such as the DCT domain.) In some systems, isolated pixels are changed in accordance with one or more bits of the binary data; in others, plural domain-related groupings of pixels (e.g. locally adjoining, or corresponding to a given DCT component) are so changed. In all cases, however, pixels have served as the ultimate carriers of the embedded data.
While pixelated imagery is a relatively recent development, line art goes back centuries. One familiar example is U.S. paper currency. On the one dollar banknote, for example, line art is used in several different ways. One is to form intricate webbing patterns around the margin of the note (generally comprised of light lines on dark background). Another is so form grayscale imagery, such as the portrait of George Washington (generally comprised of dark lines on a light background).
There are two basic ways to simulate grayscales in line art. One is to change the relative spacings of the lines to effect a lightening or darkening of an image region. FIG. 1A shows such an arrangement; area B looks darker than area A due to the closer spacings of the component lines. The other technique is to change the widths of the component linesxe2x80x94wider lines resulting in darker areas and narrower lines resulting in lighter areas. FIG. 1B shows such an arrangement. Again, area B looks darker than area A, this time due to the greater widths of the component lines. These techniques are often used together.
In my prior applications, I noted that conventional watermarking techniques are unsuitable for use with a type of line art known as vector graphics. (In vector graphics, lines are digitally described by reference to their geometry.) In particular, I noted that a change of even a single bit in a vector graphic can have substantial, unintended effects (e.g. changing a circle to a square), making the subliminal encoding of binary watermark data difficult.
In those prior applications, I proposed various solutions to this problem. One solution was to recognize that the eye is relatively insensitive to the precise placement and/or contours of a line, permitting slight modulation to effect information encoding. In particular, I noted:
xe2x80x9cThe primary example is the borders and contours between where a given line or figure is drawn or not drawn, or exactly where a bit-map changes from green to blue. In most cases, a human viewer of such graphics will be keenly aware of any attempts to xe2x80x9cmodulate signature signalsxe2x80x9d via the detailed and methodical changing of the precise contours of a graphic object. Nevertheless, such encoding of the signatures is indeed possible. The distinction between this approach and that disclosed in the bulk of this disclosure is that now the signatures must ultimately derive from what already exists in a given graphic, rather than being purely and separately created and added into a signal. This disclosure points out the possibilities here nonetheless. The basic idea is to modulate a contour, a touch right or a touch left, a touch up or a touch down, in such a way as to communicate an N-bit identification word. The locations of the changes contours would be contained in a an analogous master noise image, though now the noise would be a record of random spatial shifts one direction or another, perpendicular to a given contour. Bit values of the N-bit identification word would be encoded, and read, using the same polarity checking method between the applied change and the change recorded in the master noise image.xe2x80x9d
The present disclosure expands on these principles by reference to several illustrative embodiments.
One embodiment posits a virtual grid of points imposed on a line art image (e.g. a U.S. one dollar banknote), with the points spaced at regular intervals in vertical and horizontal directions. (The horizontal and vertical intervals need not be equal.) The virtual points may be imposed over some or all of the bill at equal vertical and horizontal spacings of 250 xcexcm. In regions of the banknote having line art, the component lines of the art snake in and amongst these virtual grid points.
Each grid point is considered to be the center of a rounded-square region. The luminance of the region is a function of the proximity of any line(s) within the boundary of the region to the region""s centerpoint, and the thickness of the line(s).
To change the luminance of the region, the contour of the line(s) is changed slightly within the region. In particular, the line is made slightly thicker to decrease luminance; or thinner to increase luminance. (Unless otherwise noted, dark lines on light backgrounds are presumed.) The ability to effect these slight changes is then employed, in accordance with known pixelation-based watermarking techniques, to encode binary data in the line art. If such a banknote is thereafter scanned by a scanner, the values of the pixel data produced by the scanner will reflect the foregoing alterations in luminance values, permitting embedded watermark data to be decoded.
In an alternative embodiment, the line widths are not changed. Instead, the positions of the lines are shifted slightly towards or away from certain virtual grid points to effect an increase or decrease in the corresponding area""s luminosity, with the same effect. Other embodiments are also detailed.
By the techniques disclosed herein, line art images can be encoded to subliminally convey binary data. This capability permits various hardware systems to recognize banknotes, and to change or limit their actions in a predetermined manner (e.g. a photocopier equipped with this capability can refuse to reproduce banknotes, or can insert forensic tracer data in the copy).
The foregoing features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.